The present invention relates to an atmospheric pressure ionization mass spectrometer, and particularly to an atmospheric pressure ionization mass spectrometer having an ionization function using molecular reaction such as atmospheric pressure ionization, chemical ionization or the like.
There is known an atmospheric pressure ionization mass spectrometer which has an ion source operating under atmospheric pressure so as to analyze sample gas (for example, as disclosed in JP-A 60-127453, JP-A 61-54144, JP-A 62-103954, JP-A 3-296659, and JP-A 4-353761).
FIG. 6 is a schematic view showing the configuration of such an atmospheric pressure ionization mass spectrometer. This atmospheric pressure ionization mass spectrometer is an apparatus for performing qualitative or quantitative analysis of NO, O.sub.2 and so on included in nitrogen gas, and, particularly, an apparatus for making measurement as to how much impure gas such as NO gas, O.sub.2 gas and so on is mixed into nitrogen in the process of producing nitrogen gas.
In FIG. 6, N.sub.2 gas, which is sample gas to be measured, is fed to a valve 2 through a stainless steel pipe 3 from a N.sub.2 gas cylinder 1. The N.sub.2 gas is introduced to an API (Atmospheric Pressure Ionization) ion source 4 through the valve 2. The N.sub.2 gas introduced into the API ion source 4 is ionized by corona discharge generated from the top end of a needle electrode 5. The ionized N.sub.2 gas reacts with impure gas such as NO, O.sub.2 and so on included in the N.sub.2 gas to thereby ionize the NO and O.sub.2 gases. This is because the ionization potential of the N.sub.2 gas is higher than that of the NO and O.sub.2 gases. The thus ionized N.sub.2 ions extract electrons from the NO and O.sub.2 gases when the N.sub.2 ions collide against the NO and O.sub.2 gases having lower ionization potential, thereby ionize the NO and O.sub.2 gases. Ions of the ionized NO and O.sub.2 gases are passed through a first small hole electrode 6, an intermediate pressure portion 7 and a second small hole electrode 8, then focused by an electrostatic lens 9, and thereafter supplied, as an ion beam, to a mass spectrometric portion 10. In the mass spectrometric portion 10, the ion beam is dispersed in accordance with mass so that NO ions and O.sub.2 ions are detected as mass spectra of the mass numbers 30 and 32 respectively.
On the other hand, in the case of quantitative measurement, N.sub.2 gas having a known NO content, for example, is introduced into the API ion source 4 as standard gas from a standard gas cylinder 12 through a valve 11. Normally, this standard gas contains NO by about 100 ppm in N.sub.2 gas. The standard gas is mixed with sample gas passed through the valve 2 immediately before it is introduced into the API ion source 4, and the mixture is introduced into the API ion source 4 to thereby be ionized by corona discharge in the same manner as mentioned above.
Each of the valves 2 and 11 has a mass flow controller 26 as shown in FIG. 7. In the mass flow controller 26, a gas flow passage is branched into two portions where a bypass 261 and a sensor 262 are disposed respectively. The sensor 262 has two self-heating resistors wound on a capillary tube, and the two resistors are connected to a bridge circuit 263. A signal from the bridge circuit 263 is supplied to a correction circuit 265 through an amplifier circuit 264.
An output voltage signal from the correction circuit 265 is supplied to a comparator/controller circuit 27 and, at the same time, supplied to a control means (not-shown). The comparator/controller circuit 27 compares a setting voltage signal supplied from the control means with the output voltage signal from the correction circuit 265, and controls a control valve 28 so that there is no difference between both the setting voltage signal and the output voltage signal.
Here, assume that the sample gas is made to flow through the valve 2 at the flow rate Qx (liter/min), and at the same time, the standard gas is made to flow through the valve 11 at the flow rate Qs (liter/min), so that the sample and standard gases are mixed with each other. In this case, for example, supposing that the density of NO in the standard gas is Cs (ppm), then the additive density of NO becomes (Qs/Qx).Cs (ppm) according to primary approximation. The additive density is adjusted by changing the flow rate Qs of the standard gas.
The standard gas is added in a plurality of stages while the quantity of sample gas is kept constant. At that time, the additive density is plotted on the abscissa, and the ionic strength is plotted on the ordinate. Assuming that the ionic strength and the additive density have a linear relationship expressed by a line y=ax+b, then the ionic strength y when the additive density x is zero, that is, the value b represents the ionic density of NO included in the sample gas. Further, the value of x, that is, Xo, when the ionic strength is zero, designates the density of NO. FIG. 8 shows this relationship. That is, the density of NO in the sample gas is expressed by the following expression (1).
Xo=b/a --(1)
The gas analysis with such an atmospheric pressure ionization mass spectrometer has the following features.
1. The number of times of collision of molecules or ions against with each other is large under atmospheric pressure. Accordingly, even a very small amount of impurities have many chances of collisions, and it is possible to perform high sensitive analysis. PA0 2. Substances lower in ionization voltage are ionized mainly. Accordingly it is possible to perform selective ionization.